Method and apparatus for adjusting channel quality indicator feedback period to increase uplink capacity

ABSTRACT

A method and apparatus for adjusting a channel quality indicator (CQI) feedback period to increase uplink capacity in a wireless communication system are disclosed. The uplink capacity is increased by reducing the uplink interference caused by CQI transmissions. A wireless transmit/receive unit (WTRU) monitors a status of downlink transmissions to the WTRU and sets the CQI feedback period based on the status of the downlink transmissions to the WTRU. A base station monitors uplink and downlink transmission needs. The base station determines the CQI feedback period of at least one WTRU based on the uplink and downlink transmission needs and sends a command to the WTRU to change the CQI feedback period of the WTRU.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. provisional application No.60/710,986 filed Aug. 24, 2005, which is incorporated by reference as iffully set forth.

FIELD OF INVENTION

The present invention is related to a wireless communication system.More particularly, the present invention is related to a method andapparatus for adjusting a channel quality indicator (CQI) feedbackperiod to increase uplink capacity in a wireless communication system.

BACKGROUND

In a wireless communication system, such as universal mobiletelecommunication services (UMTS) terrestrial radio access (UTRA), awireless transmit/receive unit (WTRU) sends a channel quality indicator(CQI), (or channel quality estimates), to a base station. The CQI isused for adaptive modulation and coding (AMC), channel sensitivescheduling, or the like. The base station determines an optimalmodulation scheme and coding rate for the WTRUs based on the reportedCQIs. The base station also uses the reported CQIs when determiningwhich WTRUs should be allowed for transmission.

The frequency of generation and transmission of the CQIs is controlledby parameters specified by a radio network controller (RNC). Theparameters are given to the WTRU through radio resource control (RRC)signaling at call setup or upon reconfiguration.

The transmission of CQIs by the WTRUs, although beneficial foroptimizing the capacity on the downlink, generates interference on theuplink. This interference may decrease the uplink capacity of thewireless communication system when the number of WTRUs that are requiredto transmit a CQI is large. Furthermore, it is often the case that thetransmission of CQIs by certain WTRUs is superfluous. Such a situationarises when a WTRU has no pending transmission on the downlink due to aperiod of inactivity at the application level.

The interference caused by the transmission of CQIs from non-activeWTRUs may be reduced by updating the CQI parameters so that the CQIs aregenerated by those WTRUs less frequently. However, this approach doesnot work well in practice because the CQI parameter update is performedthrough the RRC signaling, which is slow. By the time the CQI parameterupdate is communicated to the WTRU, the user of the WTRU may haveresumed activity, and performance would suffer until a new update issent to restore the original frequency of CQI generation.

In addition, in some circumstances it is desirable to reduce theinterference from CQI transmissions during a limited period of time inorder to increase the capacity available on the uplink when there is atemporary need for more capacity, (e.g., when one user has a largeamount of data to upload, such as a picture).

Therefore, it is desirable to provide a method to adjust the CQIfeedback period more quickly and efficiently to increase uplinkcapacity.

SUMMARY

The present invention is related to a method and apparatus for adjustinga CQI feedback period to increase uplink capacity in a wirelesscommunication system. The uplink capacity is increased by reducing theuplink interference caused by CQI transmissions. In accordance with afirst embodiment of the present invention, a WTRU monitors a status ofdownlink transmissions to the WTRU and sets the CQI feedback periodbased on the status of the downlink transmissions to the WTRU. Inaccordance with a second embodiment of the present invention, a basestation monitors uplink and downlink transmission needs. The basestation determines the CQI feedback period of at least one WTRU based onthe uplink and downlink transmission needs and sends a command to theWTRU to change the CQI feedback period of the WTRU.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram of a process for adjusting a CQI feedbackperiod in accordance with a first embodiment of the present invention.

FIG. 2 is a flow diagram of a process for adjusting a CQI feedbackperiod in accordance with a second embodiment of the present invention.

FIG. 3 is a block diagram of a WTRU which implements the process of FIG.1.

FIG. 4 is a block diagram of a base station which implements the processof FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

When referred to hereafter, the terminology “WTRU” includes but is notlimited to a user equipment, a mobile station (STA), a fixed or mobilesubscriber unit, a pager, or any other type of device capable ofoperating in a wireless environment. When referred to hereafter, theterminology “base station” includes but is not limited to a Node-B, asite controller, an access point (AP) or any other type of interfacingdevice in a wireless environment.

The features of the present invention may be incorporated into anintegrated circuit (IC) or be configured in a circuit comprising amultitude of interconnecting components.

FIG. 1 is a flow diagram of a process 100 for adjusting a CQI feedbackperiod in accordance with a first embodiment of the present invention.In accordance with the first embodiment, a WTRU autonomously adjusts theCQI feedback period based on downlink transmission status. The WTRU isinitially configured with a normal CQI feedback period, and the WTRUreports a CQI to a base station via an uplink channel every CQI feedbackperiod.

The WTRU monitors the status of downlink transmissions to the WTRU (step102). In monitoring the downlink transmission status, the WTRU maymaintain a counter for counting the number of consecutive transmissiontime intervals (TTIs) that do not include a transmission for the WTRU.For example, in high speed downlink packet access (HSDPA), the number ofTTIs that do not include a transmission for the WTRU may be determinedby detecting a valid cyclic redundancy check (CRC) on downlinktransmissions on a high speed shared control channel (HS-SCCH). Thecounter is reset when a valid downlink transmission to the WTRU isdetected, (e.g., in HSDPA, a valid CRC is detected on the HS-SCCH).

The WTRU then adjusts the CQI feedback period based on the status of thedownlink transmissions (step 104). A piece-wise function or a look-uptable (LUT) may be used to select a new CQI feedback period based on thecounter value, such that the CQI feedback period is increased as thecounter value increases, and the CQI feedback period is decreased as thecounter value decreases. An exemplary mapping scheme for mapping thecounter value to the CQI feedback period is shown in Table 1. As shownin Table 1, the increased CQI feedback periods may be a factor of thenormal CQI feedback period. The base station may monitor and detect CQIsfrom the WTRU every normal CQI feedback period regardless of the CQIfeedback period setting in the WTRU. With this scheme, it is avoidableto miss CQIs due to inconsistent CQI feedback period settings in theWTRU and the base station. The parameters in Table 1 are configurablevia a higher layer signaling, which is preferably performed at callsetup.

TABLE 1 The number of TTIs without a transmission for the WTRU CQIfeedback period  0-31 normal feedback period, P 32-63 2P  64-127 3P128-511 4P 512 or higher 5P

Alternatively, the WTRU may be given multiple CQI feedback periods, (forexample, two (2) CQI feedback periods: an active CQI feedback period andan inactive CQI feedback period), via RRC signaling, and may switchbetween the CQI feedback periods in accordance with the counter value,(i.e., the number of TTIs without a transmission for the WTRU). Forexample, when the counter value is below a threshold, the active CQIfeedback period is selected, and when the counter value is equal to orabove the threshold, the inactive CQI feedback period is selected.

After the WTRU adjusts the CQI feedback period based on the status ofthe downlink transmissions, the process 100 returns to step 102 tofurther monitor the downlink transmission status.

FIG. 2 is a flow diagram of a process 200 for adjusting a CQI feedbackperiod in accordance with a second embodiment of the present invention.In accordance with the second embodiment, a base station sends a commandto increase or decrease the CQI feedback period of the WTRUs based onuplink and downlink transmission needs. The base station increases theCQI feedback period, (which means less frequent CQI feedbacks), when theuplink transmission needs increase, while the downlink transmissionneeds can still be supported with the less frequent CQI feedbacks. Asfrequent CQI feedbacks are beneficial to downlink performance, the basestation trades downlink capacity for uplink capacity on a short termbasis.

The base station monitors uplink transmission needs and downlinktransmission needs (step 202). The uplink and downlink transmissionneeds are determined based on an amount of data buffered in each of theWTRUs for uplink transmissions and an amount of data buffered in a basestation for downlink transmissions to each of the WTRUs, respectively.The amount of data buffered in the WTRU for uplink transmission isindicated by the WTRU. For example, such indication may be given by ascheduling request, a happy bit or traffic volume measurement as in UTRARelease 6.

Alternatively, the base station may estimate the time required totransmit the data currently in the buffer of each WTRU and the timerequired to transmit the data buffered in the base station for each WTRUbased on average downlink and uplink throughput to and from each of theWTRUs.

The base station determines whether it is desirable to change the CQIfeedback period of at least one WTRU based on the uplink transmissionneeds and the downlink transmission needs (step 204). The base stationmay increase the CQI feedback period when the uplink transmission needsare high and the downlink transmission needs are low, and may decreasethe CQI feedback period, (or restore the original CQI feedback period),when the uplink transmission needs are low or the downlink transmissionneeds are high.

For example, if, for at least one WTRU, the estimated time required totransmit data in a buffer of the WTRU exceeds a pre-determinedthreshold, (i.e., the uplink transmission needs are high), it isdesirable to reduce the interference caused by CQI transmissions byincreasing the CQI feedback period. Therefore, the base stationdetermines if some or all of the downlink transmissions could affordless frequent CQI feedbacks. In order to determine this, the basestation may determine if the estimated time required to transmit thedata in the buffer of the base station on the downlink is within apre-determined threshold. If it is determined that some or all of thedownlink transmissions may afford less frequent CQI transmissions,(e.g., the estimated time required to transmit the data in the buffer ofthe base station is within the predetermined threshold), the basestation determines to increase the CQI feedback period.

If it is determined at step 204 that it is not desirable to change theCQI feedback period, the process returns to step 202 to further monitorthe uplink and downlink transmission needs. If it is determined at step204 that it is desirable to change the CQI feedback period, the basestation then sends a command to at least one WTRU to change the CQIfeedback period of the WTRU (step 206). After sending the command, theprocess 200 returns to step 202 to monitor the uplink and downlinktransmission needs.

If the base station subsequently determines that restoring the originalCQI feedback period is desirable for some or all of the WTRUs, (i.e., ifthe base station determines that the estimated time required to transmitthe data in the buffer of the base station on the downlink exceeds thepre-determined threshold, or if the base station determines that theestimated time required to transmit the data in the buffer of each ofthe WTRUs on the uplink is below the pre-determined threshold), the basestation sends a command to some or all WTRUs to restore the original CQIfeedback period of their CQI transmissions.

The command must be transmitted quickly, (e.g., within a few tens ofmilliseconds), to the concerned WTRUs or all WTRUs after a decision ismade by the base station. The command may be transmitted by any suitablemeans. For example, in UTRA Release 6, the command may be sent via anHS-SCCH. During each 2 ms TTI, the HS-SCCH includes informationnecessary for each WTRU to determine if any data will be transmitted tothe WTRU in the next TTI. The HS-SCCH includes bits for indicating achannelization code set combination for the WTRU. Currently, there areeight (8) unused bit combinations for the channelization code setcombinations. One of the 8 unused bit combinations may be used for thepurpose of sending the command to change the CQI feedback period. Forexample, one of the unused bit combinations may be used to signal anincrease of the CQI feedback period and another to signal a restorationof the original CQI feedback period.

The amount of change of the CQI feedback period in response to thecommand from the base station may be pre-determined, (e.g., by a factorof 2). Increase of the CQI feedback period by a factor of 2 means thatevery other CQI that would normally be transmitted with the originalconfiguration is now not transmitted. Alternatively, the amount ofchange of the CQI feedback period in response to the command may besignaled upon call setup or reconfiguration. For example, two sets ofCQI feedback periods may be given to the WTRU, and switched inaccordance with the command.

The information contained in a specific TTI in an HS-SCCH is normallyonly used by one specific WTRU, which is identified through bit-maskingof the CRC field with a WTRU-specific sequence, (WTRU identity (ID)). Inorder to provide a significant interference reduction on the uplinkwithin a short amount of time, it is desirable that all WTRUs monitoringa given HS-SCCH be commanded a change of the CQI feedback period at thesame time. Therefore, a special WTRU ID for all WTRUs may be used totransmit the command via the HS-SCCH.

FIG. 3 is a block diagram of a WTRU 300 which implements the process 100of FIG. 1. The WTRU 300 includes a downlink status monitor 302, a CQIfeedback controller 304 and an LUT 306 (optional). The downlink statusmonitor 302 monitors a status of downlink transmissions to the WTRU. Thedownlink status monitor 302 may include a counter 308 to count thenumber of consecutive TTIs that do not include transmissions to theWTRU. The CQI feedback controller 304 sets the CQI feedback period basedon the status of the downlink transmissions to the WTRU as statedhereinabove.

FIG. 4 is a block diagram of a base station 400 which implements theprocess 200 of FIG. 2. The base station 400 includes a monitor 402 and aCQI feedback controller 404. The monitor 402 monitors uplinktransmission needs and downlink transmission needs. The CQI feedbackcontroller 404 determines the CQI feedback period of at least one WTRUbased on the uplink transmission needs and the downlink transmissionneeds and sends a command to at least one of the WTRUs to change the CQIfeedback period.

Although the features and elements of the present invention aredescribed in the preferred embodiments in particular combinations, eachfeature or element can be used alone without the other features andelements of the preferred embodiments or in various combinations with orwithout other features and elements of the present invention.

What is claimed is:
 1. A method for adjusting channel quality indicator(CQI) feedback, the method comprising: receiving, by a wirelesstransmit/receive unit (WTRU), a radio resource control (RRC) messageincluding CQI configuration information; wherein the CQI configurationinformation indicates a number of transmission time intervals (TTIs)between CQI transmissions; transmitting CQI information in TTIs inresponse to the CQI configuration information; monitoring, by the WTRU,a physical layer downlink control channel; in response to information onthe physical layer downlink control channel indicating a CQI command andincluding a cyclic redundancy check (CRC) masked with a WTRU identityassociated with the WTRU, transmitting CQI information in a TTI inresponse to the CQI command received on the physical layer downlinkcontrol channel instead of the CQI configuration information received inthe RRC message.
 2. The method of claim 1 wherein the information on thephysical layer downlink control channel includes a single bit indicatingthe CQI command.
 3. A wireless transmit/receive unit (WTRU) comprising:at least one component configured to receive a radio resource control(RRC) message including channel quality indicator (CQI) configurationinformation; wherein the CQI configuration information indicates anumber of transmission time intervals (TTIs) between CQI transmissions;the at least one component further configured to transmit CQIinformation in TTIs in response to the CQI configuration information;the at least one component further configured to monitor a physicallayer downlink control channel; the at least one component furtherconfigured in response to the physical layer downlink control channelindicating a CQI command and including a cyclic redundancy check (CRC)masked with a WTRU identity associated with the WTRU, to transmit CQIinformation in a TTI in response to the CQI command received on thephysical layer downlink control channel instead of the CQI configurationinformation received in the RRC message.
 4. The WTRU of claim 3 whereininformation on the physical layer downlink control channel includes asingle bit indicating the CQI command.